Rudders are commonly used at the tails of aircraft to control the yaw orientation of the aircraft, for instance in combination with elevators. The rudder consists of a panel that is mounted to the tail of the aircraft. The rudder is pivoted about a generally upright axis by way of a rudder bar, so as to project partially out of a centerline of the aircraft. As a result, the drag forces caused by the displacement of the rudder will affect the yaw orientation of the aircraft.
In some very specific instances, there may be required a greater out-of-centerline orientation of the rudder. For instance, during the takeoff phase, it may occur that one of the engines of the aircraft shuts off. In such cases, the aircraft does not have sufficient velocity to oppose sufficient rudder drag forces against the one-sided thrust of the remaining operative engine. In such cases, it may be required to increase the out-of-centerline orientation of the rudder to compensate for the thrust being on a single side of the aircraft. However, once the aircraft is airborne, it may not be necessary to maintain as much of this out-of-centerline orientation, considering that the velocity of the aircraft is high enough to control the aircraft with normal out-of-centerline orientation of the rudder.
In other instance, it may be required to increase the blow down limit of the rudder. In these instances, the actuator forces controlling the rotation of the rudder have reached their maximum limit, and cannot increase the angle of the rudder. The blow down limit may be reached with high headwinds.